Plasma technology has been widely used in semiconductor integrated circuit (IC) fabrication processes, such as etching, chemical vapor deposition (CVD), and sputtering. In a plasma chamber, radiofrequency (RF) power ionizes gases to generate plasma. The plasma further releases radiation having a specific wavelength and intensity. In conventional IC fabrication processes, the plasma wavelength and intensity may be used for determining the endpoint of a process. For example, the intensity of CO having a wavelength of about 483.5 nm is continuously detected during a SiO.sub.2 etch process, and the endpoint of the process is determined when a predetermined value of intensity is detected.
However, the endpoint may be incorrectly determined if the plasma is unstable. An unstable, or abnormal, plasma condition may be caused by, for example, installation of an incorrect chamber part during maintenance, poor gas or pressure control, leakage from the chamber, unstable RF matching, gas impurity, or any combination of the above. An abnormal plasma condition, such as those discussed above, can result in defects in the processed wafers or can even damage the plasma chamber. Unfortunately, there is currently no effective method for monitoring the condition of the plasma in the plasma chamber during the plasma process. Because the plasma is not monitored in conventional processes, abnormal plasma conditions are not detected until after defective wafers have already been processed. This results in the waste of a large number of wafers. Further, continued use of a plasma chamber with abnormal or unstable plasma conditions can damage the plasma chamber or other associated equipment. Therefore, there is an unmet need in the art for an on-line plasma monitoring technique to detect abnormal or unstable plasma conditions during processing.